This invention relates to a semiconductor device, particularly a ceramic packaged or resin molded semiconductor device comprising a semiconductor element in which a .alpha.-rays shielding resin layer is formed at least on the active portion of the surface.
Semiconductor devices are usually sealed by a ceramic package or sealed by a resin (namely, resin molded). However, a ceramic material constituting the ceramic package, and the resin and an inorganic filler constituting the resin mold contain radioactive elements such as uranium, thorium or the like in a concentration of the order of several ppm. These radioactive elements emit .alpha.-rays. Therefore, the active portion of a semiconductor element is irradiated with .alpha.-rays, which cause soft errors (May T. C. and Woods M. H. "A New Physical Mechanism for Soft Error in Dynamic Memories" 1978 Int. Reliability Physics Symposium IEEE Catalogue N78CH1294-8PHY, PP 33-40).
In order to prevent soft errors of a semiconductor device due to such irradiation with .alpha.-rays, it has been disclosed in British Patent Publication No. 2,036,428A that the active portion of a semiconductor element is coated, for example, with a polyimide resin layer comprising poly(4,4'-phenylenoxy phenylene pyromellitic imide) or polyimideisoindoloquinazolinedione.
However, in some cases, these polyimide resin layers decomposed on heat treatment in sealing a ceramic package with glass, and the sealing glass portion was put outward by the pressure of the cracked gas produced, so that the seal width was decreased, resulting in insufficiency of air-tightness.
When the heat resistance of such polyimide resins was tried to be improved in order to prevent the cracked gas from being produced, since in the case of conventional poly(phenylene pyromellitic imide) or the like, the resin layer was hard, was poor in adhesion to a semiconductor element and had no flexibility, the resin layer cracked or peeled off in some cases.
Further, unless the thickness of the polyimide resin layer is larger than about 45 .mu.m, sufficient shielding against .alpha.-rays has been impossible. However, it is not easy to form a resin layer having such a thickness by a present-day coating technique other than dropping.